A series of AER-tests relate to the field of objective hearing tests which particularly are used with infants and small children, since early in life standard pure-tone audiometry, which demands active participation of the individual test subject, is not yet possible.
Several of the known test methods that record evoked responses from the auditory pathway make use of broad band click stimuli. Since a click stimulus excites a relatively broad frequency area, it is difficult to obtain frequency specific information by such stimulus. Never-the-less, click stimuli are employed in many clinical test setups especially for oto-neurological evaluations or when it is the purpose of the testing to obtain a general impression about the hearing acuity of the individual. This latter applies for instance to new born hearing screening programs. However, for a more detailed analysis of the individual hearing acuity, like for instance in audiological diagnostic evaluations, frequency specific information is in demand, especially to establish a basis for an appropriate (re)habilitation by hearing aids, cochlear implants or the like.
To this end, a series of relatively new test methods have matured. These methods are referred to as Auditory Steady-State Responses (ASSR) or by other acronyms. For a review of the ASSR—the reader is referred to e.g. Picton et al. 2003. ASSR-procedures make use of a sustained stimulus often produced as a continuous carrier waveform consisting of a single or a plurality of pure tones, a broad band or a band limited noise or the like, which is modulated either in amplitude, in frequency or in both, by specific modulation signals.
Most of the electrophysiological activity which underlies the generation of auditory evoked responses (AER) is caused by non-linear processes in individual nerve fibers, neurons or other neural units, and the AER are therefore related to the envelope of the acoustic stimulus. This is especially true for the ASSR, which consequently is composed of a combination of the fundamental frequency and its harmonics of the modulation signal. Thus, the ASSR is most readily detected and analyzed in the spectral domain rather than in the temporal domain, (Picton et al. 2003).
ASSR-stimuli have hitherto been described and defined in the temporal domain, as modulation (in amplitude, frequency or in both) of one or multiple carriers. However, several ways of designing an appropriate stimulus for an ASSR-test are described in the literature (e.g. Sturzebecher et al. 2001, John et al. 2003, Picton et al. 2003) and several patents apply to this problem. In the experimental animal ‘beat’ stimuli generated by pairs of pure tones have also been used—e.g. Dolphin, 1997.
One patent by Sturtzebecher et al. 2000 (U.S. Pat. No. 6,524,258) deals with the use of several amplitude modulated carriers having a frequency that is offset by a specific frequency difference, and where all carriers are modulated with the same modulation signal.
Another patent by John et al. 2001 deals with a broad spectrum of problems related to the recording of ASSR. Relevant to the problem described herein, the John et al. patent especially concerns the use of stimuli that may be modulated independently in amplitude and in frequency or a signal whose envelope is modulated by an exponential modulation signal—see also John et al. 2002.
It is a basic characteristic of an auditory evoked response that the magnitude of the response depends on the number of auditory units that are activated synchronously by the stimulus. This is part of the reason why a click stimulus, which simultaneously activates a broad band of frequencies, provides evoked responses that in general are larger than those obtained by narrow band or frequency specific stimuli (Eggermont, 1977).
However, in the peripheral part of the auditory pathway, the Cochlea, all frequency areas along the cochlear partition do not get excited simultaneously because of the travel time through the cochlea—from the base (high frequency area) to the apex (low frequency area) (Elberling, 1974). This has lead to the design of so-called chirp stimulus (e.g. Dau et al. 2000) where the individual frequency components of the click are time-adjusted to compensate for the cochlea travel time, so the responses from nerve fibers from different parts of the cochlear partition get time-aligned or synchronized. It was demonstrated that the chirp-stimulus generates Auditory Brainstem Responses (ABR) with larger amplitudes than those generated by the corresponding standard click stimulus.
A similar approach was used by Don et al. 1997 who demonstrated that if prior to summation, narrow-band Auditory Brainstem Responses, (ABR)—that originate from different frequency regions along the cochlear partition—are time-aligned to compensate for the individual cochlea delay, this would create a much larger stacked ABR than the broad band ABR.
In the clinic, it is important to keep test time at a minimum for the detection of an auditory response or to obtain a specific response quality. Larger response amplitudes will in general result in shorter test time, and the compensation for cochlear travel time could therefore be one way of increasing response amplitude and thereby shortening the test time.
A general problem in the recording of auditory evoked responses is how to avoid that stimulus related artifacts (being either electromagnetic or acoustic) are introduced in the recording path and here cause an interaction with the physiological response. Traditionally these problems have been minimized or eliminated by careful considerations and counter measures in the recording set-up and the proper use of shielded acoustical transducers. However, at high stimulus levels it is not always possible to get rid of the artifact.
For auditory evoked responses that are recorded using brief acoustic stimuli it is possible to use the time lag between the stimulus and the generation of the evoked response and to introduce a recording time window which only incorporates the evoked response and leaves out the stimulus artifact.
Another way which traditionally has been used is to change the polarity of every other stimulus; a method which eliminates the artifact through the averaging process which most often is applied as a means to improve the signal-to-noise ratio in order to recognize the tiny evoked responses that are buried in the physiological background noise.
For the ASSR where both stimulus and response are continuous temporal separation between stimulus artifact and response is not possible. As described above, the ASSR is readily analyzed in the frequency domain, since the response energy is located only at the fundamental frequency of the stimulus envelope (the repetition or modulation frequency) and its harmonics. Most of the ASSR energy is contained within the first six harmonics or so (Cebeulla et al. 2004), which for a repetition frequency of 100 Hz will cover the frequency range up to about 600 Hz. For an ASSR stimulus that for instance consists of a single pure tone carrier which is amplitude modulated with a low frequency pure tone the stimulus energy (and that of the artifact) will be located around the carrier frequency. For lower modulation frequencies (Gower than e.g. 100 Hz) and higher stimulus carrier frequencies (larger than e.g. 1000 Hz) the energy of the response and artifact will be spectrally separated unless frequency-aliasing occurs—like that described by e.g. Picton & John, 2004. However, for lower stimulus frequencies (e.g. 500 Hz) possible interaction between response and artifact may occur—not only covering the same spectral area but actually sharing the same frequency bins in the response spectrum.